A current source inverter is provided with a DC reactor connected to a DC source, a converter for converting DC power from the DC reactor into AC power, and a controller for performing a drive control of switching elements in the converter, and this current source inverter is characterized in that it operates on reducing fluctuations of load impedance caused by a load short or the like, since the inverter is allowed to be treated as a current source when viewed from the load side. By way of example, in the case of plasma load, the inverter operates to maintain the plasma.
The current source inverter has such an advantage as supplying current stably to the load, even when the load fluctuates as described above, it is suitable for supplying power to the plasma load where impedance varies depending on circumstances.
For example, when the plasma load changes to open-condition with arc-extinguishing of the plasma, a voltage for the plasma load increases in the current source inverter. This voltage increase acts for promoting plasma ignition, thereby facilitating the ignition. On the other hand, in the case where an arc is generated on the load side and the plasma load changes to short-condition, the current source inverter supplies a steady current to the load, and thus suppressing excessive supply of current to the plasma load. Therefore, it is possible to reduce damage on the plasma load.
FIG. 15 illustrates one configuration example of the current source inverter. In FIG. 15, the current source inverter 100 is provided with a current source step-down type chopper circuit 101, a three-phase inverter circuit 102, and a three-phase transformer 103. The current source step-down type chopper circuit 101 performs chopper control of a switching element Q1, thereby stepping down DC, being inputted from an AC source and a rectifier circuit not illustrated, smoothing the current in a DC reactor LF1, and inputting the current into the three-phase inverter circuit 102.
As a chopper circuit for performing DC-AC conversion, a current source step-down and -up type chopper circuit may be employed, instead of the current source step-down type chopper circuit 101.
The three-phase inverter circuit 102 controls at a predetermined timing, arc striking and arc extinguishing of the switching elements QR, QS, QT, QX, QY, and QZ, thereby causing commutation between the elements, so as to supply AC power to the three-phase transformer 103.
This current source inverter has a problem that when all the switching elements break the passage of electric current, there is a possibility that current from the DC reactor applies overvoltage on the switching elements and causes element destruction. There is another problem that at the time of commutation, current and voltage are generated on the switching elements, and this may cause the element destruction.
There are known following techniques so as to prevent such destruction of the switching elements due to an accident of load short; a technique for detecting load current to obtain a current-carrying period, and controlling the switching elements during this current-carrying period, and a technique for detecting the current flowing in the switching elements and controlling the switching elements based on the current being detected (see Patent Document 1).
There is also known a technique which is proposed to solve problems of surge voltage occurrence and switching loss due to current interruption, and this technique obtains a current overlap time when the current in the switching elements being a commutation source becomes zero at a zero cross point of the load voltage, and starts the commutation in the switching element being a target of the commutation at a point of time earlier than the zero cross point of the load voltage, by the obtained current overlap time (see Patent Document 2).
FIG. 16 and FIG. 17 illustrate the switching loss at the time of commutation in the circuit operation shown in FIG. 15. FIG. 16 illustrates the case where the commutation is performed, without overlapping of the ON state between the switching element being the commutation source and the switching element being the commutation target. In this example, the switching element QR is assumed as the commutation source, the switching element QS is assumed as the commutation target, and gate pulse signals GR and GS set those elements to be the ON state, respectively (FIG. 16A and 16B). Since a falling edge of the gate pulse signal GR agrees with a rising edge of the gate pulse signal GS, commutation between the switching elements is performed without overlapping. Here, as shown in FIG. 16C and 16E, the time constants of the current IQR and the voltage VQR (drain-to-source voltage of the switching element) flowing in the switching element QR at the commutation source, change at the time of being turned OFF, under the influence of wiring inductance, element capacity, load inductance, and the like. Therefore, they do not achieve ZCS and ZVS at the time of commutation, and a switching loss occurs. In addition, surge voltage is generated, and this may cause damage on the switching element.
Even though an overlap period is provided, during when the ON state of the switching element being the commutation source and the ON state of the switching element being the commutation target overlap each other at the time of commutation, as shown in FIG. 17C and 17E, they do not achieve ZCS and ZVS at the time of commutation, and the switching loss occurs.
There is known a resonant inverter as a soft switching inverter for reducing the switching loss.
The resonant inverter connects a freewheeling diode and a resonant capacitor with the switching element in parallel, and a resonant circuit is made up of the resonant capacitor, the resonant inductance, and the switching element connected to the resonant circuit. Discharging and charging of the resonant capacitor by the resonant current from the resonant circuit, and conduction of the freewheeling diode achieve ZVS (zero voltage switching) and ZCS (zero current switching) in the switching element (e.g., Patent Document 2).
Since the resonant circuit has a configuration where the resonant capacitor is connected in parallel to the switching element, there is a problem that the capacitor increases in volume. In order to solve the problem above, it is suggested that an auxiliary circuit made up of auxiliary switching elements forms the resonant circuit (Patent Document 3).